All great suggestions... But when you mentioned science the thing that came to mind is -- host a homeschool science fair! I run the science fair at my DDs home learning center and it's great fun. You can have regular "classes" to help kids learn about the scientific method, asking good questions, and doing experiments, and then help with putting together boards. You don't have to judge the fair, but you can hand out certificates of participation to everyone.
It's perfect because the kids can participate at whatever level they are at and they won't slow each other down.
